1. Field of the Invention
The present invention relates to an optical communication transmitter, an optical communication receiver, an optical communication system, and a communication apparatus for communicating a signal using a white-light-emitting diode.
2. Description of the Related Technology
In recent years, white-light-emitting diodes (white LEDs) have become increasingly popular. White LEDs are applicable to a variety of applications, such as lighting equipment, car lamps, and backlights of liquid crystal displays. Compared with other white light sources (such as fluorescent lamps), white LEDs have a very quick response when powering on and powering off. By using this feature, a system that uses the white LED lighting as a data transmission capability has been proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-318836).
In the data transmission system using a white LED, white light emitted from the LED is used as a data transmission medium. The intensity of light emitted from the LED is modulated in accordance with the transmission data. At a receiving end, the light is received by means of a photoelectric converter (O/E converter), such as a photodiode (PD), and is detected to achieve the data transmission.
The types of white LED are classified into three groups by a light emission method (refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-290335).
A first light emission method of the white LED is to combine a blue LED and a yellow fluorescent material. In this method, a fluorescent material such as YAG (yttrium aluminum garnet) is disposed around a blue LED, and the fluorescent material and the blue LED are packaged into one component. The light from the blue LED disposed at the center excites the fluorescent material so that the fluorescent material emits light that is complementary to blue light (mainly yellow light). By combining this fluorescent light and the original blue light from the blue LED, pseudo-white light is produced. Hereinafter, this type of white LED is referred to as a “blue-light-excited white LED”. The advantages of the blue-light-excited white LED are as follows: (a) The energy use efficiency is high compared with other methods, and therefore, a high luminance can be easily obtained; and (b) The structure is simple, and therefore, the manufacturing cost is low. However, the blue-light-excited white LED has a disadvantage in that the blue-light-excited white LED has poor color rendering properties. As used herein, the term “color rendering properties” is referred to as the appearance of colors of an object when illuminated by light from a given source. As the color is closer to that under natural light, the color rendering properties become higher.
A second light emission method of the white LED is to combine a UV LED and a fluorescent material for emitting three primary light components RGB (red, green, and blue). The fluorescent material for emitting the three primary light components RGB is disposed around the UV LED, and the fluorescent material and the UV LED are packaged into one component. The light from the UV LED disposed at the center excites the fluorescent material so that the fluorescent material emits the three primary light components RGB. Thus, white light is produced. Hereinafter, this type of white LED is referred to as a “UV-excited white LED”. The UV-excited white LED has an advantage in that the UV-excited white LED has good color rendering properties. However, the UV-excited white LED has a disadvantage in that (a) The energy use efficiency is low compared with the blue-light-excited white LED, and therefore, it is difficult to produce high luminance; and (b) A high driving voltage is required to emit UV light.
A third light emission method of the white LED is to combine a red LED, a green LED, and a blue LED into one package. By emitting three primary color light from the corresponding LEDs at the same time, white light is produced. Hereinafter, this type of white LED is referred to as a “three-color-emitting white LED”.
The three-color-emitting white LED has an advantage in that, like the UV-excited white LED, the three-color-emitting white LED has good color rendering properties. However, the three-color-emitting white LED has a disadvantage in that the three-color-emitting white LED is expensive compared with the other methods since three types of LED are integrated into one package.
The features of data transmission are described next when each type of white LED is used for data transmission.
When data transmission is performed using the blue-light-excited white LED, which is currently widely available in the marketplace, the transmission speed of the blue-light-excited white LED is limited to several Mbps since the response time of the light emitted from the fluorescent material is slow. To solve this problem, Japanese Unexamined Patent Application Publication No. 2003-318836, for example, describes a technique for decreasing response time in which a color filter that transmits substantially only blue light is disposed at the front of an O/E converter so as to remove a light component that is emitted from the fluorescent material and that has a slow response time. However, even when this technique is applied, the response speed is only increased up to several tens of Mbps.
When data transmission is performed using the UV-excited white LED, the transmission speed is limited to several Mbps due to the same reason as that for the blue-light-excited white LED. In addition, the driving voltage of the LED is high, and therefore, it is difficult to design a driving circuit.
When data transmission is performed using the three-color-emitting white LED, high-speed data transmission can be provided since the three-color-emitting white LED has no fluorescent light components and different signals can be transmitted from the different LEDs (multiple-wavelength transmission) (refer to, for example, T. Taguchi, et. al, “Future Prospect and Application of White LED Lighting System Technologies,” CMC Publishing CO., LTD, July, 2003). However, since each of the LEDs is expensive, the system cost is disadvantageously increased.
As described above, when performing data transmission using the white LED, the transmission speed can be increased by using the three-color-emitting white LED. However, since the three-color-emitting white LED is expensive, the use of the three-color-emitting white LED is not always appropriate in terms of the versatility.
In addition, when performing data transmission using the blue-light-excited white LED, the limit of the transmission speed can be increased only up to several tens of Mbps even when a color filter that blocks light emitted from the fluorescent material having a slow response time is used as described in Japanese Unexamined Patent Application Publication No. 2003-318836.
FIG. 8 illustrates the configuration of a data transmission system including a transmitter 10 and a receiver 20 using the blue-light-excited white LED according to a known technology. In this system, a modulator 11 modulates light output from a white LED 12 using a modulation method, such as on-off keying (OOK). Thus, the white light emitted from the white LED 12 blinks. The blinking state is detected by a light detector 21 of the receiver 20 disposed far from the transmitter 10 and is demodulated by a demodulator 22. Thus, a signal can be transmitted from the transmitter 10 to the receiver 20. However, in such a configuration, if the signal input to the white LED 12 is turned on and off at high speed, the distortion of the waveform occurs due to the delay of the response time of light output from the fluorescent material, as shown in FIG. 9. Accordingly, intersymbol interference occurs, which impairs high-speed transmission using the blue-light-excited white LED.
FIG. 10 illustrates another configuration of a data transmission system including a transmitter 10 and a receiver 20 using the blue-light-excited white LED. In this system, an optical filter 23 that transmits substantially only the wavelength of light from the blue LED (hereinafter referred to as a “blue filter”) is provided to the receiver 20. The blue filter 23 eliminates light emitted from a fluorescent material having a slow response time from the light signal. Therefore, substantially only the light emitted from the blue LED enters the light detector 21. As a result, data transmission faster than that in the configuration shown in FIG. 8 can be achieved. However, the cutoff frequency of existing LEDs is several tens of MHz at the highest. If the signal is OOK-modulated for a transmission speed corresponding to a frequency higher than the cutoff frequency, distortion as shown in FIG. 9 is generated in the output light signal, and therefore, intersymbol interference occurs. Accordingly, the highest transmission speed is also limited.
In addition, in data transmission using the UV-excited white LED, like data transmission using the blue-light exited white LED, the slow data transmission speed due to the slow response time of light emitted from a fluorescent material cannot be avoided. Furthermore, since the high driving voltage is required, it is disadvantageously difficult to design a driving circuit for the UV-excited white LED.
Still furthermore, although a method for decreasing a response time of light emitted from a fluorescent material by improving the characteristics of the fluorescent material has been proposed, it is difficult to obtain a desired luminance. Also, the cost of the fluorescent material increases.
Accordingly, from a viewpoint of a system configuration, it is desirable that high-speed signal transmission is achieved using the blue-light-excited white LED that is versatile and inexpensive.